Terahertz SiGe imager sees through clothes

PORTLAND, Ore.  Silicon-germanium (SiGe) RF chips now in lab prototype form could one day be used in millimeter-wavelength W-band imaging devices sensitive enough to "see" through clothing to reveal concealed weapons.

EEs from the University of California at San Diego (UCSD) presented their design at the IEEE RFIC Symposium in Boston on June 9. The chip operates in the terahertz range (1 THz = 1,000 GHz) to provide X-ray-like vision, but using safe, naturally occurring millimeter wavelengths. The designers said the chip could be produced using inexpensive silicon processing techniques.

"Our chip can resolve images down to a millimeter scale, enabling us to identify very small objects that are on someone's body," said professor Gabriel Rebeiz, a designer of millimeter-wave RFICs, phased arrays and microelectromechanical system (MEMS) chips, in whose UCSD lab the SiGe terahertz RFICs were built.

In addition to their envisioned use for security applications, terahertz imagers could aid in navigation when storm or dust-cloud conditions limit visibility, as well as transfer enormous amounts of data over secure line-of-sight connections.

Because silicon-based semiconductors do not ordinarily operate above 10 GHz, imager designers today use expensive gallium arsenide or indium phosphide amplifiers. At the RFIC Symposium, however, several designs using CMOS and BiCMOS processes were described in addition to UCSD's SiGe solution, promising less costly processes that could be run on standard silicon fabrication equipment.

"We should be able to bring the costs of those sorts of systems down, perhaps even to handheld scanners," said Jason May, an EE doctoral candidate who works at Rebeiz' UCSD lab.

The lab's terahertz imager prototype implemented a W-band square-law detector in a commercial SiGe process and included an integrated low-noise amplifier and switch on a chip that consumed just 0.26 square millimeters. The chip operated at 94 GHz consuming 29 milliamps from a 1.2-volt supply.